1. Field of the Invention
This invention relates to a fuel tank venting system for vehicles such as motor cars, which operates to secure ventilation between the interior and exterior of the fuel tank even when the fuel tank is tilted.
2. Prior Art
Conventionally, a fuel tank venting system of this kind is known, for example, from Japanese Utility Model Publication (Kokoku) No. 51-3290, which includes a front vent pipe having a front inclined portion and a rear inclined portion, the front inclined portion having an open end thereof arranged above the fuel liquid surface within the fuel tank, a vertically extending increased-volume vent pipe connected to the rear inclined portion, a rear vent pipe extending rearward of the fuel tank with an open end thereof located above the fuel liquid surface within the fuel tank at a rear side of the fuel tank and the other end thereof connected to an upper end of the increased-volume vent pipe, and a drain pipe connected at an end thereof to an upper portion of the increased-volume vent pipe and communicating at the other end thereof with the atmosphere or the like via a check valve (hereinafter referred to as "the first conventional system").
According to the first conventional system, when the fuel tank is tilted forwardly of the vehicle such that a front side of the fuel tank lowers, the open end of the rear vent pipe becomes positioned above the fuel liquid surface. Therefore, when the temperature of the fuel tank rises to increase the pressure of air within the fuel tank, part of the air flows through the rear vent pipe and the drain pipe to open the check valve to be emitted into the atmosphere or the like. As a result, no fuel flows out of the fuel tank.
On the other hand, when the fuel tank is tilted rearwardly of the vehicle such that the rear side of the fuel tank lowers, the open end of the rear vent pipe becomes blocked with the fuel. Then, part of the air within the fuel tank is moved into the increased-volume vent pipe as its pressure rises and stays there. Then, upon reaching a lower end of the increased-volume vent pipe, the air floats upward in the form of bubbles in the increased-volume vent pipe without pushing upward fuel in the pipe which has been introduced into the pipe via the open end of the front vent pipe and then is emitted into the atmosphere or the like via the drain pipe.
Further, another fuel tank venting system of this kind is conventionally known, which includes an anti-overcharging valve for prevention of fuel overcharging formed by a float valve, and a cut valve formed by a float valve integrally connected to a check valve arranged in a charging passage extending between a fuel tank and a canister, and a spring urging the float valve in a direction of closing the same, the float valves being arranged at one end of the charging passage in the fuel tank at an upper central location thereof, and a two-way valve arranged in the charging passage at a location downstream of the anti-overcharging valve (hereinafter referred to as "the second conventional system"). The sum of the set pressure of the check valve and the set pressure of the two-way valve is set to a value larger than a head pressure of the fuel tank (pressure difference between the fuel liquid surface within the fuel tank and a fuel inlet end of a filler pipe of the fuel tank) assumed when the fuel tank is fully charged and hence the anti-overcharging valve is closed, to thereby restrain the head pressure when the fuel tank is fully charged, thus preventing overcharging of the fuel tank with fuel.
According to the first conventional system, however, as an element which can control the pressure within the fuel tank, only a single valve i.e., the check valve is arranged in the exhaust pipe, which therefore has a high set pressure (valve opening pressure) to prevent overcharging of the fuel tank, which renders it difficult to satisfy a puff-loss condition which is prescribed by United States fuel vapor emission standards (i.e., the pressure within the fuel tank should be controlled below a predetermined value lower than the valve opening pressure in a designated test mode).
On the other hand, according to the second conventional system, if the fuel tank is of a thin and oblong type, only the single cut valve arranged at the upper central location in the fuel tank cannot satisfactorily achieve gas-liquid separation performance. This is because the cut valve becomes immersed in the fuel when the fuel tank is tilted or inclined, and therefore when the pressure within the fuel tank rises due to heat or the like, air within the fuel tank cannot escape, which causes fuel to flow through the cut valve toward the engine. If two cut valves are provided in the fuel tank to overcome this inconvenience, these cut valves must be separately arranged on opposite sides of the fuel tank to satisfactorily achieve the gas-liquid separation performance, which inevitably leads to provision of two anti-overcharging valves on the opposite sides of the fuel tank. As a result, if refueling is made at a gas station having a sloping floor, for example, fuel is poured into the fuel tank to such a level as high as the level of one of the cut valves (which are integrally connected to the anti-overcharging valves) which is then positioned at a higher level. Thus, the fuel tank is overcharged with an amount of fuel much larger than an amount charged when the fuel tank is in a horizontal position. In such an overcharged state, if the pressure within the fuel tank rises, there is a fear that fuel can flow through the cut valve, which is then positioned at a lower level, toward the engine, which hinders satisfactory achievement of the gas-liquid separation performance.